In production and automation technology, serial bus systems are increasingly used in which the remotely arranged devices of machine peripherals such as I/O modules, transducers, drives, valves and operator terminals communicate with automation, engineering and visual display systems via an efficient real-time communication system. In this arrangement, all users are networked via a serial bus, preferably via a field bus, the data exchange via the bus being in general carried out on the basis of the master-slave principle.
The active bus users on the bus system, the control devices, as a rule, possess a bus access authorization and determine the data transfer on the bus. The active bus users are called the master units in the serial bus system. In contrast, passive bus users are, as a rule, machine peripheral devices. They do not receive a bus access authorization, i.e. they are allowed to acknowledge only received information signals or transfer information signals to a master unit upon request by the latter. Passive bus users are called slave units in the serial bus system.
To avoid complex cabling, field bus systems having a master-slave structure are generally arranged in ring topology, all bus users being connected to a ring-shaped transmission path. An information signal generated by the master unit is fed into the ring-shaped transmission path by the master unit and successively passes through the slave units serially connected to the ring-shaped transmission path and is then received again and evaluated by the master unit.
As a rule, the information signals are organized by the master unit into data packets which are composed of control data and useful data, preferably using the Ethernet standard which provides for data packets having a length of up to 1500 bytes with a transmission speed which, at the same time, may be as high as 100 Mbit/sec. Each of the slave units connected to the ring-shaped transmission path processes the useful data intended for it of the Ethernet messages fed into the ring-shaped transmission path by the master unit.
As a rule, the master-slave communication systems having a ring structure are configured in such a manner that the master unit has a transmitter unit as data injection point and a receiving unit as data extraction point. The individual slave units are then connected together on the transmission path to form a ring, wherein each user is connected to two neighbors and the first and last user in the chain is connected to the master unit. The data packets are thereby transmitted in one direction starting from the master unit via its transmitter unit to the first connected slave unit and from there to the next one, until the last slave unit in the ring has been reached, and then back to the receiving unit of the master unit.
Each slave unit has a first terminal for receiving the circulating data packets from the previous user and a second terminal for forwarding to the following user, a processing unit being arranged between the two terminals in order to process the data packets passing through the slave unit.
A central demand for master-slave communication systems, particularly when they are used in production and process automation, is a high fault tolerance, that is to say the capability of the communication system to maintain the required function, i.e., for example, the production of a work piece, in spite of the occurrence of faults. In this context, faults in the communication system which must be overcome without impairment of the process are, in addition to faults in the data packets, also the failure of entire users, in particular the master unit in the transmission path or, respectively, an interruption in the transmission path, for example due to physical separation of the transmission medium.